Growing cement instead of manufacturing it. The idea comes from a team of U.S. researchers at the University of Colorado Boulder, who, inspired by coral reef algae, have come up with the idea of imitating their natural calcification processes in a new, sustainable and carbon neutral method of cement production.
If all the cement used by the construction industry in the world was replaced by this new biogenic limestone cement, the researchers state, as much as 2 billion tons of CO2 emissions per year could be avoided, while another 250 million could be stored in the new material, thanks to the absorption properties of the algae.

Cultivating limestone like coral reef microalgae

In the best tradition of scientific research, the idea for biogenic cement first came up outside the lab. In 2017, Wil Srubar, an associate professor of civil and environmental engineering at the University of Colorado Boulder, was snorkeling in Thailand in the middle of a coral reef. Observing the beautiful calcium carbonate structures created over hundreds of thousands of years by the reef's microalgae, suddenly a thought sparked: "If Nature can grow limestone, why can't we?"
The idea can be placed in that strand of experimentation on neo-materials christened Growing Design. It is an approach that radically revolutionizes the idea of building, replacing fabrication with natural growth: basically, we cultivate matter.
"Algae are typically thought of as resources, as raw materials, but we started looking at their incredible properties under a different light," Srubar explains. He and his team then began cultivating coccolithophores, white microalgae that through photosynthesis sequester and store carbon dioxide in mineral form. Nourished by sunlight, seawater and dissolved carbon dioxide, these microscopic algae are able to produce large amounts of calcium carbonate at a significantly faster rate than coral reefs.

Biogenic cement to cut 2 billion tons of CO2

Limestone (consisting of calcium carbonate) is the basis for the manufacture of Portland cement, the most commonly used cement in the world, and concrete. Limestone rocks are mined in quarries and then baked at very high temperatures, thus generating, both during extraction and during the industrial process, enormous amounts of CO2. It is estimated that, annually, the entire cement industry produces 2.8 billion tons of carbon emissions, between 5 and 9 percent (depending on estimates) of the global total.
However, by replacing limestone extracted from quarries with organically grown limestone, a zero-emission process for making Portland cement could be created, according to Srubar's team, since the CO2 emitted into the atmosphere would be offset by that captured by the algae themselves.
“If all cement-based construction around the world was replaced with biogenic limestone cement”, the researchers write, “each year a whopping 2 gigatons of carbon dioxide would no longer be pumped into the atmosphere and more than 250 million additional tons of carbon dioxide would be pulled out of the atmosphere and stored in these materials”.

A plug-and-play material

The solution, according to Srubar, would be easily scalable: "Biogenic cement," he explains, "is a plug-and-play material, since it fits into traditional cement industry processes."
There would also be no problems with space or weather conditions for growing coccolithophores. In fact, these tiny organisms are incredibly resilient and can live in salt water as well as fresh water, at high and low temperatures: perfect, in short, to be grown just about anywhere, even in an urban waterhole. Srubar's team calculated that 1 to 2 million acres (between 4,000 and 8,000 square kilometers) of open waterholes would be enough to cover all the cement needs of the United States, basically the equivalent of 1 percent of the land used for corn crops.
The estimated numbers are so compelling that the U.S. Department of Energy has awarded a $3.2 million grant to the University of Colorado Boulder team to develop the project. Srubar and colleagues are already working on commercializing the process. "For the cement and concrete industry, now is the time to solve this very wicked problem," they comment, "And we believe we have one of the best solutions, if not the best solution, to address it."

Image: Marek Okon (Unsplash)